Foams and cellular solids are currently used in a wide variety of commercial and military applications, including armor, structural load bearing members, aircraft, cars, thermal and noise insulation, heat transfer, and catalysis. As such, the current state of the art in cellular solids is a very broad topic. Research areas include decreasing costs, increasing strengths, stiffnesses, energy absorption, heat exchange, catalytic capacity, biocompatibility, exploring foaming methods for new materials, hollow sphere structures, and high temperature suitable foams. (See, e.g., L P Lefebvre, J Banhart, D C Dunand, Adv. Eng. Mat. 10 (2008) 775; Y Boonyongmaneerat, C A Schuh, D C Dunand, Scripta Mater. 59 (2008) 336; X Xue, Y Zhao, JOM 63 (2011) 43; A Rabiei, L J Vendra, Mater. Lett. 63 (2009) 533; O Reutter, J Sauerhering, T Fend, R Pitz-Paal, S Angel, Adv. Eng. Mat. 10 (2008) 812; R Singh, PD Lee, JR Jones, G Poologasundarampillai, T Post, T C Lindley, R J Dashwood, Acta Biomater. 6 (2010) 4596; A H Brothers, D C Dunand, MRS Bull. 32 (2007) 639; U Jehring, P Quadbeck, HD Bohm, G Stephani, in Porous Metals and Metallic Foams, DEStech Publications Inc., Lancaster, Pa., 2008, pp. 165-168; and Y Boonyongmaneerat, D C Dunand, Adv. Eng. Mat. 10 (2008) 379; and G Walther, B Klöden, T Weissgärber, B Kieback, A Böhm, D Naumann, S Saberi, L Timberg in Porous Metals and Metallic Foams, DEStech Publications Inc., Lancaster, Pa., 2008, pp. 125-128, the disclosures of each of which are incorporated herein by reference.) Aluminum foams dominate the metallic foam literature and fabrication methods include the use of TiH2 and CaCO3 blowing agents and modifications to existing methods to achieve better properties. (See, LP Lefebvre, (2007), cited above.)
In addition to these standard materials, some research has been done on bonded hollow sphere structures. In conventional techniques, hollow spheres are made by coating sacrificial spheres with crystalline metals and thermally or chemically removing the sacrificial material or atomizing metallic melts. Hollow crystalline metal spheres can then be sintered together or “glued” with a binder material. (See, e.g., WS Sanders, L J Gibson, Mat. Sci. Eng. A 347 (2003) 70, the disclosure of which is incorporated herein by reference.) High strength cellular structures have been achieved with steel alloys (U. Jehring, (2008) cited above) and other crystalline metals (LP Lefebvre, (2007) cited above), but these individual methods are difficult to engineer, and may only be used with a limited number of materials.
As would be understood, regardless of the specific type of foam chosen, the properties required of that foam depend on the particular application. For example, foams used for armor and energy absorbing structures should be as light as possible while absorbing the maximum energy at a given plateau stress. (See, e.g., M F Ashby, Phil. Trans. R. Soc. A 364 (2006) 15, the disclosure of which is incorporated herein by reference.) In turn, foams used in load bearing applications should be designed for minimum weight at a given load. (See, M F Ashby, L U Tianjian, Science in China Series B 46 (2003) 521, the disclosure of which is incorporated herein by reference.) Likewise, sandwich panels and foam core structures (used in aircraft and race cars for example) require maximum stiffness, while minimizing weight. (See, L J Gibson, M F Ashby, Cellular Solids Structure and Properties, Cambridge University Press, New York, N.Y., 1997, pp. 55-61, 345-385, the disclosure of which is incorporated herein by reference.) Meanwhile, closed cell foams can be used for thermal, vibration, and noise insulation. Open cell foams, on the other hand, allow exposure to a large surface area to fluids flowing through them, which can be used for heat transfer and catalysis. (See, M F Ashby, A G Evans, N A Fleck, L J Gibson, J W Hutchinson, H N G Wadley, Metal Foams: A Design Guide, Butterworth-Heinemann, Woburn, Mass., 2000, pp. 113-188; and LP Lefebvre, J Banhart, D C Dunand, Adv. Eng. Mat. 10 (2008) 775, the disclosures of which are incorporated herein by reference.)
The general examples above currently require the selection of appropriate materials for each application, in a time consuming manner. First, the requirements for the application are quantified. Then, a “property profile” is developed which details the characteristics a material would need to meet the requirements. This selection process relies on compendiums of materials to see if a known material matches the property profile. If no material exists, new alloys must be invented or research and development must be performed to address the problem.
Within the amorphous metallic field, many patents on methods for foaming amorphous metallic glasses have been granted. U.S. Pat. No. 5,384,203 discusses a method similar to those found in U.S. Pat. Nos. 4,099,961 and 5,281,251, wherein a blowing agent is injected into the molten mixture and the material is foamed above the solidus temperature. Likewise, U.S. Pat. Nos. 7,073,560 and 7,621,314 both teach methods to introduce blowing agents into the metallic glass forming alloy in the molten state and then expand the bubbles upon cooling from the melt but above Tg or by cooling to a solid, reheating the alloy above Tg and expanding the bubbles at that time. Meanwhile, U.S. Pat. No. 7,597,840 teaches a method of making a foam precursor by consolidating amorphous powders around finely dispersed particles of blowing agent and foaming that mixture above Tg.
However, despite the extensive research, the scientific literature reveals limited success in making high porosity foams from metallic glasses. (See, Brothers, Dunand. Scripta Mater. 54 p 513, 2006, the disclosure of which is incorporated herein by reference.) Expensive Pd and Pt glass forming alloys are one example of high porosity foams. Boron Oxide Hydrate is dissolved in the amorphous melt to form a “pre-foam” and the mixture is expanded at T>Tg to form high strength, highly porous structures. (See, Demetriou, Hanan, Veazey, Di Michiel, Lenoir, Üstündag, Johnson. Adv. Mater. 19 p1957, 2007; and Wang, Demetriou, Schramm, Liaw, Johnson. J. Appl. Phys. 108 p023505, 2010, the disclosures of which are incorporated herein by reference.) Fe based metallic glasses and Zr based metallic glasses have also been foamed using different methods, but porosity is usually lower than that obtainable for Pd based bulk metallic glass (BMG) forming alloys. (See, Demetriou, Duan, Veazey, De Blauwe, Johnson. Scripta Mater. 57 p9, 2007; and Brothers, Scheunemann, DeFouw, Dunand. Scripta Mater. 52 p335, 2005, the disclosures of which are incorporated herein by reference.) Nowhere is there provided a method that allows for the formation of foams from a wide-variety of amorphous materials in a manner that also provides a way to uniquely tailor the cell size, wall thickness, internal cell pressure, and material strength.
Accordingly, a need exists to find a novel approach that could produce foams and cellular materials with a range of densities, strengths, and stiffnesses to meet these varied applications and needs.